Process for preparing 2-alkyne 1-acetals

ABSTRACT

Process for preparing 2-alkyne 1-acetals of the general formula I,  
                 
where the radicals have the following meanings: 
         R 1  is hydrogen, C 1 -C 20 -alkyl, C 2 -C 20 -alkenyl, C 2 -C 20 -alkynyl, C 3 -C 12 -cycloalkyl, C 4 -C 20 -cycloalkylalkyl, C 4 -C 10 -aryl, with these radicals optionally being able to be substituted by hydroxy, halogen, C 1 -C 6 -alkoxy, (C 1 -C 6 -alkoxy)carbonyl, carboxyl or nitrile groups,    R 2  is C 1 -C 6 -alkyl or a radical of the general formula II  
                 
 
where R 1  is as defined above, 
   R 3  has the same meaning as the radical R 2 , 
 
by electrochemically oxidizing a compound of the general formula III  
                 
 
where the radical R 1  has the same meaning as in the general formula I, in the presence of C 1 -C 6 -alkyl alcohols (alcohol A) or as such.

The present invention relates to a process for preparing 2-alkyne 1-acetals.

Nonelectrochemical processes for preparing 2-alkyne 1-acetals are known, for example, from U.S. Pat. No. 2,879,305, in which 2-alkyne 1-aldehydes are acetalated with an alcohol.

Journal of Electroanalytical Chemistry (1994), 371(1-2), 167-77, and Electrochimica Acta (1993), 38(10), 1337-44, disclose electrochemical processes carried out in aqueous solution in which a 2-alkyn-1-ol is oxidized. However, targeted formation of the acetal is not described.

Chemische Berichte (1954), 87, 668-76, and Surface Science (2000), 457(1-2), 178-184; DE-A1-2409117 disclose electrochemical processes in which a 2-alkyn-1-ol is oxidized. The formation of the corresponding aldehydes, acids or of C0₂ is described there.

It is known that a saturated alcohol (methanol) (cf. Bulletin of the Chemical Society of Japan (1991), 64(3), 796-800; Journal of Organic Chemistry (1991), 56(7), 2416-21; Synlett (1990), (1), 57-8 (with mediator)) or an allyl ether alcohol (cf. Tetrahedron, 43, 24 (1987), 5797-5806) can be oxidized to an acetal in an electrochemical process.

It is an object of the invention to provide an electrochemical process for preparing 2-alkyne 1-acetals economically and, in particular, in high product yields and with high selectivity.

We have accordingly found a process for preparing 2-alkyne 1-acetals of the general formula I,

where the radicals have the following meanings:

-   -   R¹ is hydrogen, C₁-C₂₀-alkyl, C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl,         C₃-C₁₂-cycloalkyl, C₄-C₂₀-cycloalkylalkyl, C₄-C₁₀-aryl, with         these radicals optionally being able to be substituted by         hydroxy, halogen, C₁-C₆-alkoxy, (C₁-C₆-alkoxy)carbonyl, carboxyl         or nitrile groups,     -   R² is C₁-C₆-alkyl or a radical of the general formula II         where R¹ is as defined above,     -   R³ has the same meaning as the radical R²,     -   by electrochemically oxidizing a compound of the general formula         III         where the radical has the same meaning as in the general formula         I, in the presence of C₁-C₆-alkyl alcohols (alcohol A) or as         such.

In general, compounds of the general formula I in which the radicals R² and R³ are identical are prepared.

In the case of compounds of the general formula I in which R² is a radical of the general formula II, preference is given to R³ also being a radical of the general formula II.

In the case of compounds of the general formula I in which R² and R³ are each a radical of the general formula II, the radical R¹ which occurs therein 3 times preferably has the same meaning on each occurrence. In the preparation, the compound of the general formula III is used as such, i.e. no alcohol A is used but instead the starting compound of the general formula III performs the function of the alcohol A which acetalates the oxidation product of the compound of the general formula III.

Particular preference is given to preparing compounds of the general formula I in which the radicals R² and R³ are each methyl. Accordingly, methanol is then used as alcohol A.

With regard to the radical R¹, preference is given to compounds of the general formula I in which is hydrogen, a C₁-C₆-alkyl radical or a C₁-C₆-alkyl radical substituted by a hydroxyl group. Accordingly, compounds of the general formula 11 in which the radical R¹ is hydrogen, a C₁-C₆-alkyl radical or a C₁-C₆-alkyl radical substituted by a hydroxyl group are then used as starting compounds.

Very particular preference is given to preparing compounds of the general formula I in which R¹ is hydrogen and the radicals R² and R³ are each methyl. The starting materials used then are 2-propyn-1-ol as compound of the general formula III and methanol as alcohol A.

A further compound of the general formula I which is very particularly preferably prepared is 1,1,4,4-tetramethoxybut-2-yne. Starting materials used then are 2-butyne-1,4-diol as compound of the general formula III and methanol as alcohol A.

In the electrolyte, the alcohols A and the compound of the general formula III are generally used in a molar ratio of 2:1, based on the alcoholic hydroxyl groups present in the compound of the formula III, or the alcohol A is used in excess and then serves simultaneously as solvent or diluent for the compound of the general formula III and the compound of the general formula I formed.

When using a compound of the general formula III bearing 2 or more alcoholic hydroxyl groups, preference is given to using more than 2 mol (preferably from 5 to 20 mol) of the alcohol A per mole of hydroxyl groups borne by the compound of the general formula III in order to avoid oligomerization.

If appropriate, customary cosolvents are added to the electrolysis solution. These are the inert solvents having a high oxidation potential which are customarily used in organic chemistry. Examples which may be mentioned are dimethyl carbonate and propylene carbonate.

Water is also suitable in principle as cosolvent; the proportion of water in the electrolyte is preferably less than 20% by weight.

Electrolyte salts present in the electrolysis solution are generally alkali metal, tetra(C₁-C₆-alkyl)ammonium, preferably tri(C₁-C₆-alkyl)methylammonium, salts. Suitable counterions are sulfate, hydrogensulfate, alkylsulfates, arylsulfates, halides, phosphates, carbonates, alkylphosphates, alkylcarbonates, nitrate, alkoxides, tetrafluoroborate and perchlorate.

Furthermore, the acids derived from the abovementioned anions are also possible as electrolyte salts, i.e., for example, sulfuric acid, sulfonic acids and carboxylic acids.

Further suitable electrolyte salts are ionic liquids. Suitable ionic liquids are described in “Ionic Liquids in Synthesis”, Editors Peter Wasserscheid, Tom Welton, Verlag Wiley VCH, 2003, Chapters 1 to 3.

The process of the invention can be carried out in all customary types of divided or undivided electrolysis cells. It is preferably carried out continuously in undivided flow-through cells.

Bipolar capillary gap cells or plate stack cells in which the electrodes are configured as plates and arranged in a parallel fashion (cf. Ullmann's Encyclopedia of Industrial Chemistry, 1999 electronic release, Sixth Edition, VCH-Verlag Weinheim, Volume Electrochemistry, Chapter 3.5. special cell designs, and Chapter 5, Organic Electrochemistry, Subchapter 5.4.3.2 Cell Design), are very particularly suitable. Graphite is preferred as electrode material.

When the process is carried out continuously, the feed rate of the starting materials is generally chosen so that the weight ratio of the compounds of the general formula 11 used to the compounds of the general formula I formed in the electrolyte is from 10:1 to 0.05:1.

The current densities at which the process is carried out are generally in the range from 1 to 1000 mA/cm², preferably from 10 to 100 mA/cm². The process is generally carried out at atmospheric pressure. Higher pressures are preferably employed when the process is to be carried out at relatively high temperatures, so as to avoid boiling of the starting compounds or the solvent.

Suitable anode materials are, for example, noble metals such as platinum or metal oxides such as ruthenium or chromium oxide or mixed oxides of the RuO_(x)TiO_(x) type and also diamond electrodes. Preference is given to graphite or carbon electrodes.

Possible cathode materials are, for example, iron, steel, stainless steel, nickel or noble metals such as platinum and also graphite or carbon materials and also diamond electrodes. Preference is given to the system graphite as anode and cathode and also graphite as anode and nickel, stainless steel or steel as cathode.

After the reaction is complete, the electrolyte solution is worked up by conventional separation methods. For this purpose, the electrolysis solution is in general firstly distilled and the individual compounds are obtained separately in the form of different fractions. Further purification can be carried out, for example, by crystallization, extraction, distillation or chromatography.

Experimental Part

EXAMPLE 1

Example of Undivided Operation:

An undivided plate stack cell having graphite anodes and steel cathodes was used. 160 g of 2-propyn-1-ol in 640 g of methanol were reacted with 5.3 g of sulfuric acid at a temperature of 20° C. for 19 hours. The electrolysis was carried out at 3.4 A/dm2 and an amount of charge of 2 F based on the 2-propyn-1-ol used was passed through the cell. The output from the electrolysis contained 8.8 GC-% by area of 1,1-dimethoxy-2-propyne (conversion: 49%, yield: 30%).

EXAMPLE 2

Example of Divided Operation:

A divided parallel plate cell having a graphite anode and steel cathode was used. 401 g of 2-propynol in 1400 g of methanol and 38 g of MTBS (methyltributylammonium methylsulfate) in the anode space and 941 g of methanol and 24 g of MTBS in the cathode space were reacted at a temperature of 20° C. for 38 hours. The electrolysis was carried out at 3.1 A/dm² and an amount of charge of 2 F based on the 2-propyn-1-ol used was passed through the cell. The anolyte contained 8.3 GC-% by area of 1,1-dimethoxy-2-propyne (conversion: 91%, yield 19%).

EXAMPLE 3

Example of Divided Operation:

A divided parallel plate cell having a graphite anode and steel cathode was used. 13 g of 2-butyne-1,4-diol in 117 g of methanol and 3.8 g of MTBS (methyltributylammonium methylsulfate) in the anode space and 130 g of methanol and 3.8 g of MTBS in the cathode space were reacted at a temperature of 19° C. for 14 hours. The electrolysis was carried out at 3.4 A/dm² and an amount of charge of 4 F based on the 2-butyne-1,4-diol used was passed through the cell. After the electrolysis, anolyte and catholyte were combined and a yield (according to GC-% by area) of 1,1-dimethoxy-2-butyn4-ol of 53% and of 1,1,4,4-tetramethoxy-2-butyne of 20% was obtained at a conversion of 100%. 

1. A process for preparing 2-alkyne 1-acetals of the general formula I,

where the radicals have the following meanings: R¹ is hydrogen, C₁-C₂₀-alkyl, C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₃-C₁₂-cycloalkyl, C₄-C₂₀-cycloalkylalkyl, C₄-C₁₀-aryl, with these radicals optionally being able to be substituted by hydroxy, halogen, C₁-C₆-alkoxy, (C₁-C₆-alkoxy)carbonyl, carboxyl or nitrile groups, R² is C₁-C₆-alkyl or a radical of the general formula II

where R¹ is as defined above, R³ has the same meaning as the radical R, by electrochemically oxidizing a compound of the general formula III

where the radical R¹ has the same meaning as in the general formula I, in the presence of C₁-C₆-alkyl alcohols (alcohol A) or as such.
 2. The process according to claim 1, wherein 2-propyn-1-ol or 2-butyne-1,4-diol is used as compound of the general formula III and methanol is used as alcohol A.
 3. The process according to claim 1, wherein the process is carried out in an electrolyte comprising sulfuric acid, sulfonic acids or carboxylic acids.
 4. The process according to claim 1, wherein the process is carried out in an electrolyte comprising less than 20% by weight of water.
 5. The process according to claim 1, wherein the process is carried out in an electrolyte comprising one or more compounds selected from the group consisting of sodium, potassium, lithium, iron, tetra(C₁-C₆-alkyl)ammonium salts with sulfate, hydrogensulfate, alkylsulfates, arylsulfates, halides, phosphates, carbonates, alkylphosphates, alkylcarbonates, nitrate, alkoxides, tetrafluoroborate or perchlorate as counterion or ionic liquids as electrolyte salt.
 6. The process according to claim 1, wherein the process is carried out in a bipolar capillary gap cell or plate stack cell or in a divided electrolysis cell.
 7. The process according to claim 1, wherein at least 2 mol of the alcohol A are used per mole of hydroxyl groups borne by the compound of the general formula III when using a compound of the general formula III bearing 2 or more alcoholic hydroxyl groups.
 8. The process according to claim 1, wherein from 5 to 20 mol of the alcohol A are used per mole of hydroxyl groups borne by the compound of the general formula III when using a compound of the general formula III bearing 2 or more alcoholic hydroxyl groups.
 9. The process as claimed in claim 1, wherein the process is carried out with current densities in the range from 1 to 1000 mA/cm².
 10. The process as claimed in claim 1, wherein the process is carried out with current densities in the range from 10 to 100 mA/cm². 